The present invention relates to the field of signal sampling. In particular, this invention relates to an apparatus and method for measuring a signal with a signal sampler.
A Josephson junction sampling circuit is disclosed in U.S. Pat. No. 4,401,900 of Sadeg M. Faris (the '900 patent"). Operation of a Josephson junction sampler requires a feedback current to balance out the unknown signal current. A known signal measurement system employs a Josephson junction sampling gate, a transition or level detector, and an analog integrator in a feedback loop. One type of known Josephson junction sampler runs at a constant clock rate.
In a known signal measurement system employing analog integration, feedback gain is directly proportional to the clock period of the system, and such a direct proportionality is not desirable if the clock rate of the system is variable over a wide range of frequencies. A variable clock rate can occur, for example, if the system clock is synchronized to external signals with different clock rates. A known signal measurement system using analog integration may experience problems with a low repetition rate signal. If gain is dependent on the sampling rate, then one problem is that the gain of the analog integrator increases as the frequency of the applied signal decreases, thereby leading to problems with the feedback loop of the system. If, however, the grain of the system employing analog integration is kept constant by known methods, at low signal frequencies drift and offset in the integrator may nevertheless be a problem.
The known analog system employing analog integration thus does not have the flexibility of adapting to varying clock rates, and therefore is not a good general purpose system.
Even when clock rates are fixed, however, the known analog system does not have the characteristic of relatively rapid signal acquisition. Furthermore, in order for the known analog system to be able to send signals to a computer, an analog-to-digital converter is required, and an analog-to-digital converter is generally slower and more expensive to implement in a system than a digital-to-analog converter.